1. Field of the Invention
This invention relates to a lithium secondary battery and in particular to a lithium secondary battery having an improved negative electrode containing a carbonaceous material.
2. Description of the Related Art
In recent years, a nonaqueous electrolyte secondary battery using lithium as a negative electrode active material has been attracting attentions as a high energy density battery. Among such nonaqueous electrolyte secondary batteries, a primary battery using manganese dioxide (MnO2), carbon fluoride [(CF2)n] or thionyl chloride (SOCl2) as a positive electrode active material is already widely used as a power source of a timepiece or an electric calculator, or as a backup battery of a memory.
In addition, as the sizes and weights of various types of electronic equipment, such as VTR devices or communication equipments have been decreased, a demand for a secondary battery having a high energy density which can be suitably used as a power source of these equipments has been increased, and therefore, the nonaqueous electrolyte secondary battery has been actively studied.
For example, the preparation of a nonaqueous electrolyte secondary battery has been studied wherein the negative electrode thereof is constituted by lithium, an electrolyte is constituted by a nonaqueous electrolyte which can be prepared by dissolving an electrolytic salt such as LiClO4, LiBF4 or LiAsF6 in a nonaqueous solvent such as propylene carbonate (PC), 1,2-dimethoxyethane (DME), xcex3-butyrolactone (xcex3-BL) or tetrahydrofuran (THF), or constituted by a lithium ion-conductive solid electrolyte, and a positive electrode active material is constituted by a compound which is capable of topochemically reacting with lithium such as TiS2, MoS2, V2O5; V6O13 and MnO2 for instance.
However, the lithium secondary battery as mentioned above has not been put into practical use yet. This is mainly because the charge/discharge efficiency of the battery is low and the number of charge/discharge times (or cycle life) thereof is still insufficient. The cause for this poor performance is assumed to be ascribed to the fact that lithium constituting the negative electrode is degraded due to a reaction with a nonaqueous electrolyte. Namely, lithium dissolved in the nonaqueous electrolyte in the form of lithium ions during the discharging reacts with a solvent as it is precipitated at the moment of charging thereby causing the surface of lithium to be partially inactivated. Therefore, when the charge/discharge is repeated, lithium is precipitated in the form of dendrites or small spheres, or is separated from the collector.
On the other hand, the employment as a negative electrode of a lithium alloy represented by a general formula of LixA (where A is a metal such as Al) is now studied. The negative electrode constituted by this lithium alloy is large in absorption/desorption ratio of lithium ions per unit volume of the negative electrode and hence high in capacity. However, since this negative electrode expands and shrinks as the lithium ions are absorbed therein and desorbed therefrom, the structure of the lithium alloy would be destroyed in the repetition of the absorption/desorption of lithium ions. Therefore, a lithium secondary battery provided with this negative electrode is defective in that the charge/discharge life thereof is short.
For these reasons, there has been proposed to employ, as a negative electrode for a lithium secondary battery, a carbonaceous material which is capable of absorbing or desorbing lithium ions such as coke, sintered resin, carbon fibers or pyrolytic epitaxial carbon so as to prevent the degradation in performance of a negative electrode that may be brought about by a reaction between lithium and a nonaqueous electrolyte or by the precipitation of dendrite.
It is now considered that the charge/discharge of a negative electrode comprising the aforementioned carbonaceous materials is mainly performed by the movement of lithium ions entering into or getting out of an interface between layers constituting a laminate structure of carbon planes formed of carbon atoms (a graphite structure) in the carbonaceous material. For this reason, it is required to employ, as a negative electrode for a lithium secondary battery, a carbonaceous material which is developed more or less in graphitization. However, when a carbonaceous material to be obtained by pulverizing a macro-crystal developed in graphitization is employed as a negative electrode in a nonaqueous electrolyte, the decomposition of the nonaqueous electrolyte will be caused, resulting in the deterioration of capacity and charge/discharge efficiency of the battery. Moreover, as the charge/discharge cycle is repeated, the deterioration of capacity will be accelerated, thus deteriorating the cycle life of the battery.
There has been studied to promote the capacity of a negative electrode comprising the aforementioned carbonaceous materials. For example, a coin type battery provided with a cathode comprising a sintered body of an epoxy novolak resin having pores 7.4 to 8.8 angstroms in diameter which have been formed by means of a small angle X-ray scattering method and with an anode formed of a lithium foil is disclosed in Table II of J. Electrochem. Soc., Vol. 142, No. 11, November 1995, p3668-3677.
Further, a coin type lithium secondary battery provided with a cathode comprising a pyrolytic epitaxial carbon 3.47 angstroms in d002 (as measured by means of X-ray diffraction) and containing 1.8% of Si atom, and with an anode formed of a lithium foil, the capacity per weight of the battery being 363 mAh/g, is disclosed in J. Electrochem. Soc., Vol. 142, No. 2, February 1995, p326-332.
Accordingly, the object of the present invention is to provide a lithium secondary battery comprising an improved negative electrode comprising a carbonaceous material, which is improved in discharge capacity as well as in cycle life.
Namely, according to the present invention, there is provided a lithium secondary battery comprising a positive electrode, a negative electrode comprising a carbonaceous material which is capable of absorbing and desorbing lithium ions, and a nonaqueous electrolyte;
wherein the carbonaceous material has a region of amorphous carbon structure and a region of graphite structure, and the carbonaceous material has a true density of 1.8 g/cm3 or more and a peak in powder X-ray diffraction which corresponds to not more than 0.340 nm in an interplanar spacing d002 derived from (002) reflection.
According to the present invention, there is further provided a lithium secondary battery comprising a positive electrode, a negative electrode comprising a carbonaceous material which is capable of absorbing and desorbing lithium ions, and a nonaqueous electrolyte;
wherein the carbonaceous material has a region of amorphous carbon structure and a region of graphite structure;
the carbonaceous material has a true density of 1.8 g/cm3 or more and a peak in powder X-ray diffraction which corresponds to not more than 0.340 nm in an interplanar spacing d002 derived from (002) reflection; and
the region of amorphous carbon structure has pores 0.1 to 20 nm in diameter as measured by means of a small angle X-ray scattering method.
According to the present invention, there is further provided a lithium secondary battery comprising a positive electrode, a negative electrode comprising a carbonaceous material which is capable of absorbing and desorbing lithium ions, and a nonaqueous electrolyte;
wherein the carbonaceous material has a region of amorphous carbon structure and a region of graphite structure;
the carbonaceous material has a true density of 1.8 g/cm3 or more and a peak in powder X-ray diffraction which corresponds to not more than 0.340 nm in an interplanar spacing d002 derived from (002) reflection; and
at least one of the interplanar spacings d002 derived from (002) reflection in the region of amorphous carbon structure is 0.370 nm or more.
Further, according to the present invention, there is provided a lithium secondary battery comprising a positive electrode, a negative electrode comprising a carbonaceous material which is capable of absorbing and desorbing lithium ions, and a nonaqueous electrolyte;
wherein the carbonaceous material comprises at least one kind of elements M selected from the group consisting of Mg, Al, Si, Ca, Sn and Pb and has a peak in powder X-ray diffraction which corresponds to not more than 0.344 nm in an interplanar spacing d002 derived from (002) reflection.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.